1. Field of the Invention
The present invention pertains to a method for predictively calculating a cardiac signal course, and to a control method based thereon for the delivery of a stimulation pulse by an implantable cardiac device.
2. Background Art
Regarding the background of the invention, it needs to be explained that various pathological conditions of the heart are characterized by significantly propagating wave fronts of the electrical excitement of the myocardium. The occurrence of an atrial fibrillation (hereinafter: AF), for instance, is explained by tie propagation of wave fronts on the atrial myocardium. This wave propagation is reflected in the potential that is measured locally at a bipolar electrode.
In accordance with relevant medical findings, it is known to apply a stimulation pulse with the aid of an electrode during the so-called xe2x80x9cexcitable gapxe2x80x9dxe2x80x94i.e., the short period before a new wave front characterizing a fibrillation condition arrives at the location of the electrode and after the local refraction time has elapsedxe2x80x94which effects a sort of xe2x80x9cresetxe2x80x9d for the entire cardiac muscle apparatus and stops the fibrillation. This method must be distinguished from conventional defibrillation methods, however, which can react only to the arrival of the wave front, because it requires less defibrillation energy and prevents side effects, such as the triggering of ventricular tachycardia or of a post-defibrillatory bradycardia.
Various approaches for the treatment of tachycardic conditions of the heart have already become known from the prior amt. EP 0 830 876 A1, for instance, deals with an implantable device for early detection and suppression of tachycardia in the heart in which a microelectrode away that is in contact with the cardiac muscle tissue detects the stimulus conduction potentials in the cardiac muscle tissue. A measuring device determines the refractory time of the cardiac muscle cells in the monitored cardiac region and the stimulus conduction velocity in the monitored cardiac muscle region. The product value of the refractory time and stimulus conduction velocity is representative for the tachycardia risk. If the product value falls short of a certain tachycardia threshold value, this signals a condition of the heart in risk of tachycardia. If such a condition has been detected, an anti-tachycardia stimulation pulse may be delivered with the aid of a stimulation arrangement.
This prior art has the shortcoming that, due to the microelectrode array that needs to be provided, the implantable cardiac device revealed there cannot be implemented simply by equipping a pacemaker or defibrillator with otherwise customary external characteristics with die corresponding control technology.
In order to be able to work with a conventional implant design, ways must be found whereby the excitable gap in the atrium is determinable and whereby the local potential curve at the location of the electrode is predictable. In this context, it needs to be noted that the entire spatial and temporal distribution of the local potentials over the atrium, or information on the inner degrees of freedom of the myocardium cells, are not available.
As an approach to solving the above problems the present invention now proposes a method for the predictive calculation of a cardiac signal course, the essential process steps of which are
scanning of the time progression of a cardiac signal during a certain scanning period,
calculation of model coefficients according to the method of autoregressive modeling from the scanning values of the scanning period,
predictive calculation of anticipated signal values following the scanning period during a prediction period according to the method of autoregressive modeling based on the scanning values and model coefficients, with the signal values calculated iteratively, in each case by entering the last predicted signal value as the last scanning value during the autoregressive modeling.
In this approach, the invention is based on the realization that the atrial fibrillation, as an example for a pathological condition of the heart, represents an irregular process, which appears at a local electrode as they are used in pacemakers or defibrillators, partly as an irregular potential course, partly in the form of organized potential structures. These observed irregularities raise the question whether an atrial fibrillation must be considered chaotic due to the numerous non-linearities in biological systems, and whether corresponding analysis and control methods should, therefore, be used. However, up to now, the occurrence of chaotic conditions in fibrillation episodes has not been proven. On the contrary, preliminary tests during the development of the method according to the present application, for example with the aid of the non-linear approximation of measured signals and determination of the so-called Lyapunov exponents, and also a qualitative study of measured signals, have not revealed any compelling insights indicating the presence of a deterministic exponential divergence of the fibrillation potentials. Specifically, frequent transitions between periodic segment and irregular intervals can be detected during acute atrial fibrillation. This is an indication that both a strong linear component, as well as stochastic influences are present in the case of an AF potential course and propagation. Even though the future identification of chaotic contributions, which could then be calculated predictively according to the invention with a non-linear autoregressive model, to the atrial fibrillation cannot be ruled out in this context, based on the current state of the scientific research it is not possible to derive any method for predicting the potential curve during atrial fibrillation and/or for a therapeutically effective control of implantable cardiac devices in the sense of an active termination of the fibrillation condition from die speculation concerning chaotic behavior. However, this qualification does not apply for the prediction of the fibrillation initialization with the aid of the essentially autonomously determined sequences prior to the actual occurrence of an atrial fibrillation.
In this sense, a cardiac signal such as, e.g., an EKG signal, is scanned during a certain scanning period, and the appropriate measuring points are recorded. Model coefficients are calculated from the scanning values of the scanning period according to the method of autoregressive modeling, and anticipated signal values following a scanning period are, in turn, precalculated according to the method of autoregressive modeling during a certain prediction period based on the scanning values ad model coefficients. These signal values now indicate, for example, the approach of an excitation front toward the scanning electrode so that an anti-tachycardia pulse can be delivered by the implant at a physiologically meaningful moment prior to the calculated arrival of the wave front
Compared to other conceivable mathematical methods, the linear autoregressive prediction represents the most promising approach for the precalculation of the atrial potential course, since the concept essentially is very simple and takes into account especially periodic linear components, which dominate in AF potentials.
Preferred method characteristics, the content and meaning of which will be explained in more detail below based on an embodiment presented by way of example.
Further preferred embodiments of the invention pertain to a method for controlling the delivery of a stimulation pulse by an implantable cardiac device, whereby an abnormal cardiac signal value curve can be calculated in advance with the aid of the prediction method of the basic invention, and whereby a stimulation pulse counteracting same is to be delivered accordingly within the prediction period. Specifically, the implantable device is controlled such that for an atrial fibrillation the stimulation pulse is applied in the excitable gap prior to the arrival of a precalculated wave front at the excitation electrode that delivers the pulse
A further preferred embodiment of the invention pertains to the implantable cardiac device as such, which, in addition to the commonly present control unit, measuring electrode and stimulation electrode, incorporates a computing device for the predictive calculation of a cardiac signal course according to the invention, and a stimulation unit, which can be operated with the control method according to the invention, for the electrical actuation of the stimulation electrode.